1. Field of the Invention
The present claimed invention relates to the field of semiconductor wafer fabrication. More specifically, the present claimed invention relates to the measurement of the distance from the backside of a semiconductor wafer to a nozzle of an edge bead removal system.
2. Prior Art
During conventional applications of photoresist coatings to semiconductor wafers, a "coater" system is used. One part of the coater system is a flat, circular, disk-shaped, rotating vacuum chuck having a diameter slightly less than that of a semiconductor wafer. The vacuum chuck is used to hold and rotate a semiconductor wafer during the photoresist application process. The vacuum chuck is oriented such that a semiconductor wafer placed thereon resides in a level horizontal plane. In operation, the backside or inactive surface of a semiconductor wafer is placed onto the vacuum chuck. The vacuum chuck applies a suction or negative pressure to the backside of the semiconductor wafer to hold the semiconductor wafer on the vacuum chuck.
In standard coater system 3, an axle extends downward from the vacuum chuck and is powered by motors to rotate the vacuum chuck. The axle of the vacuum chuck has a collar placed thereover. Attached to the collar is a nozzle which extends upwardly with the tip of the nozzle located in a position which will be in close proximity to the outer edge of the backside of a semiconductor wafer when a semiconductor wafer is placed on the vacuum chuck. A set screw on the collar allows the position of the collar to be adjusted so that the gap distance, the distance of the nozzle from the outer edge of the backside of a semiconductor wafer, can be varied. The vacuum chuck, collar, and nozzle are all enclosed within a shroud. The shroud collects, in a drain cup, excess fluids which are generated during the photoresist application process.
While the semiconductor wafer is being rotated on the vacuum chuck, a desired amount of liquid photoresist is applied to the center of the semiconductor wafer. While the semiconductor wafer is rotating, the photoresist material spreads radially outward from the center of the semiconductor wafer towards the edge of the semiconductor wafer such that the entire top or active surface of the wafer is coated with a layer of photoresist. However, excess amounts of photoresist tend to accumulate and form a mound or bead of photoresist on the outer edge of the semiconductor wafer. In order to eliminate the "edge bead" of photoresist, a nozzle which dispenses a solvent referred to as edge bead removal fluid, is used. During the dispersal of the edge bead removal solvent, the vacuum chuck, collar, and nozzle are commonly referred to as an edge bead removal system. The edge bead removal fluid is dispersed from the nozzle towards the outer edge of the backside of the semiconductor wafer.
Although the edge bead removal fluid is directed at the backside of the semiconductor wafer, due to the viscous nature of the edge bead removal fluid, under proper conditions the fluid will "curl" around to the top surface of the semiconductor wafer and remove the bead of excess photoresist material. However, the effectiveness of the edge bead removal system is highly dependent upon the edge bead removal gap distance separating the nozzle from the outer edge of the backside of the semiconductor wafer.
In order to set the gap distance, the vacuum chuck, collar, and nozzle must be lifted higher than the shroud so that the vacuum chuck, collar, and nozzle are visible to the user. Standard coater systems allow the vacuum chuck, collar, and nozzle to be lifted to accommodate adjustments to the collar and gap distance. The user then estimates or "eyeballs" the gap distance and makes adjustments as are deemed necessary. After each adjustment, test runs of semiconductor wafers are made to determine the effectiveness of the edge bead removal system. Repeated adjustments and test runs are made until an acceptable result is obtained. As a result, considerable time is spent determining optimum gap distance and considerable cost in test semiconductor wafers is incurred.
In addition to adjusting the gap distance, changes may be made to the pressure at which the edge bead removal fluid is dispersed from the nozzle, and the speed, revolutions per minute, at which the semiconductor wafer is rotated during the edge bead removal process. Unfortunately, when a user attempts to correct the gap distance to maximize edge bead removal system effectiveness, prior adjustments made to the fluid dispersal pressure, and changes in the speed of rotation of the semiconductor wafer must be taken into account. Furthermore, variances in estimated optimum gap distance between different users must also be taken into account when trying to properly set-up or correct edge bead removal gap distances.
Thus, the need has arisen to quickly obtain optimum edge bead removal system effectiveness, which does not require repeated eyeballing and adjustments, does not waste costly semiconductor wafers on repeated test runs, does not require adjustments to edge bead removal fluid dispersal pressures, does not require changes to semiconductor wafer rotation speed, and which provides uniformity of results even among different subsequent users.